The primary goal of this research is to characterize quantitatively the mechanism of human accommodation and age-dependent accommodative loss (presbyopia) in adult emmetropes. Accommodation, the process by which the human eye focuses on near objects, occurs through controlled changes in the shape of the crystalline lens. It is generally held that, as the ciliary muscle contracts, the lens goes through an elastic or visoelastic relaxation that results in an increased thickness along the polar axis, a shallowing of the anterior chamber, and increased thickness along the curvature on the anterior and posterior lens surfaces; the latter two events act to increase the overall refractive power of the globe. With increasing age, accommodative amplitude decreases such that the near point gradually recedes, leading generally by the age of 50 yr to the need for optical prostheses. The causes of this age-dependent degradation of accommodative range have been attributed variously to degeneration of the ciliary muscle, growth and/or hardening of the crystalline lens, liquefication of the vitreous, and changes in the geometric relationship between the lens and ciliary muscles through the zonules. We plan to use the data we have collected on 149 adult emmetropic human subjects between the ages of 18 and 70 yr, which includes results from pachymetry, keratometry, ultrasonography, and Scheimpflug slit-lamp photography, to characterize the aging of the human eye, especially the lens, and both the process of accommodation and the development of presbyopia. Statistical analysis of these data will allow identification and characterization of the interaction of age-dependent and -independent parameters in accommodation. Concurrent characterization of lens elastic properties using fresh post-mortem human lenses will directly test the hypothesis of lens stiffening with age. These data will be used in the application of two new computer-based accommodation models to the study of the aging of the accommodation process and the roles of lens-associated structures in this process. These data can also be used to study - and eventually resolve - the "lens paradox".